The invention relates to an apparatus for removing solid particles, especially soot particles, from the exhaust gas of an internal combustion engine, in particular a Diesel engine. The apparatus has a separator and a disposal device. The separator divides the flow of exhaust gas into a largely particle-free primary flow and a particle-enriched secondary flow, which exit via separate outlets. The disposal device, in turn, has a combustion chamber with an inlet opening connected to the outlet for the secondary flow of exhaust gas, a pilot burner in the combustion chamber produces a burnoff flame that burns off the solid particles, and an outlet opening for removing the gaseous products of the burnoff.
In an apparatus of this kind, also known as an exhaust scrubber, known from German Offenlegungsschrift No. 35 26 074, the separator comprises an agglomerator, also known as an electric filter tube or an electrostatic soot shunt, and a centrifugal filter or cyclone downstream of the agglomerator in the flow of exhaust gases.
In the agglomerator, there is an electrical high-voltage field, in which the solid particles become electrically charged, which causes them to coagulate into relatively large clumps, which because of their relatively great weight are readily removed from the exhaust gas flow mechanically. The mechanical filtering takes place in the centrifugal filter or cyclone, to which the exhaust gases containing the clumps is delivered at a relatively high tangential flow velocity. A rotational flow is brought about in the centrifugal filter, causing the heavy clumps to strike the outer walls and move spirally downward, from whence they are delivered to the disposal device, along with a small portion of the exhaust gas flow, forming a carrier flow, in the form of a particle enriched secondary exhaust gas flow. As the core flow, the majority of the exhaust gas flow, which is largely free of particles, leaves the centrifugal filter centrally and is delivered to the engine exhaust system as the primary exhaust gas flow. The secondary exhaust gas flow, which is heavily laden with soot and other solid clumps, amounts to approximately 1% of the largely particle-free primary exhaust gas flow.
In the known apparatus, the disposal device is designed as a combustion device and comprises a combustion chamber and a pilot burner. The inlet fitting for the secondary exhaust gas flow, embodied as a plunger tube, discharges freely into the interior of the combustion chamber, directly upstream of an overflow opening in a chamber wall dividing the pilot burner from the actual combustion chamber. Via the overflow opening, burning fuel-air mixture from the pilot burner is introduced into the combustion chamber. When the disposal device is started up, a glow element ignites the fuel-air mixture electrically, after which the flame continues to burn automatically during the controlled delivery of fuel-air mixture. The flame surrounds the end of the plunger tube and burns in the combustion chamber, along with the solid particles introduced via the plunger tube. The products of combustion of the solid particles and the other residual gases, which together can be called a gaseous burnoff product, are removed coaxially with the plunger tube via the outlet opening.
Test have shown that under unsteady engine operation conditions, which are particularly typical in engines used to drive motor vehicles, only moderate efficiency is possible with this kind of disposal device having so-called direct soot combustion. This can be ascribed particularly to the short dwell time of the soot or other solid particles in the combustion chamber. Moreover, the short dwell time dictates a very high burnoff temperature of approximately 1000.degree. C., which makes the pilot burner and combustion chamber expensive because of the materials required. Since the disposal device has no provision for storage of the entering solid particles, which instead are combusted directly, the disposal device must be continuously on, during the entire time the engine is in operation, so the required fuel consumption is not inconsiderable. In engines of varying power, the disposal device must be adapted to the individual engine, so that the combustion chamber temperature required for optimal soot combustion can be adhered to. Because of the varying exhaust gas quantities and temperatures, the engineering work involved makes this adaptation expensive.
In another known apparatus for removing solid particles from engine exhaust gas (German Offenlegungsschrift No. 31 21 274), the entire flow of exhaust gas is directed through a filter built into the engine exhaust system. The filter retains only the larger solid particles, however, which makes the disposal device relatively disadvantageous. In a combustion chamber provided on the inlet side of the filter, a pilot burner produces a flame acting upon the surface of the filter. The disposal device is operated intermittently and is turned on only once the filter has become soiled to a predetermined extent. Although compared with the first disposal device described above, the intermittent operation would mean relatively high fuel economy, it still requires bringing the entire flow of exhaust gas to the required burnoff temperature, rather than only the extremely small secondary exhaust gas flow. The burner power required is substantially greater, which not only makes the fuel consumption at least as high as with the other device described, but also requires large pilot burners. A filter disposed in the exhaust gas flow is also at a relatively high risk of filter destruction if the filter becomes overly heavily soiled, for instance from temporary failure of the burner or from increased soot in the exhaust gas, and the filter material is overheated by the ensuing high rate of soot decomposition in the burnoff process. Nor should the danger that the filter will become plugged by incombustible solid particles, which rather severely limits the service life of the filter, be underestimated.
In another known exhaust scrubber (German Offenlegungsschrift No. 34 24 196), the particle-enriched secondary exhaust gas flow is again delivered to a disposal device, in which the combustible solid particles are burned completely. The disposal device has a combustion chamber, into which the secondary exhaust gas flow is introduced axially. The combustion chamber is provided with an electric heating element through which the secondary exhaust gas flows with an admixture of air. Downstream of the heating element is a filter that traps only the incombustible solid particles contained in the gaseous combustion products. The heating element is electrically heated permanently during the entire period of engine operation. Such electric heating has the disadvantage of requiring the handling of high currents, as high as 83 A for 1000 W of heating output in a 12 V system. Electric heating is accordingly suitable only for relatively small exhaust gas scrubbers, and then the engine needs a much larger generator.